Two-stroke air-cooled engines are used for portable working machines such as bush cutters, chain saws, and power blowers (Patent Document 1). In development of the two-stroke air-cooled engines, efforts are made to comply with environmental regulations. A typical example thereof is a leading-air type two-stroke engine (Patent Documents 2 to 5). A leading-air type engine is also referred to as a “stratified scavenging engine.”
The leading-air type two-stroke air-cooled engine introduces an air into a combustion chamber in an early phase of a scavenging stroke and subsequently introduces a fuel-air mixture in a crank chamber into a combustion chamber. This kind of an engine includes a scavenging passage communicating with the combustion chamber and the crank chamber. The scavenging passage is filled with an air from an upper part thereof. The air is supplied to the upper part of the scavenging passage through a piston groove (Patent Documents 2 to 5).
The leading-air type two-stroke air-cooled engine introduces the air accumulated in the scavenging passage into the combustion chamber in the early phase of the scavenging stroke. By using this leading-air in a scavenging stroke, an HC component in exhaust gas can advantageously be reduced.
FIG. 6 accompanying this description corresponds to FIG. 3 of Patent Document 3. Reference numeral 1 shown in FIG. 6 denotes a leading-air type two-stroke air-cooled engine. The engine 1 is a single-cylinder engine.
Reference numeral 2 denotes an air-cooling fan. The engine 1 is forcibly cooled by the air-cooling fan 2. An air-cooling fan is included not only in the shown engine but also in two-stroke air-cooled engines employed in portable working machines. A cylinder 6 housing a piston 4 is made of an aluminum alloy excellent in heat transference.
The cylinder 6 houses the piston 4 in a reciprocable manner. The piston 4 makes up a combustion chamber 8. The piston 4 is coupled through a connecting rod 10 to a crankshaft 12, and engine output is output through the crankshaft 12.
The air-cooling fan 2 is attached to one end part 12a of the crankshaft 12. For example, in the case of a chain saw, the engine output is output from the other end 12b of the crankshaft 12. In the case of a bush cutter, the air-cooling fan 2 is attached to the one end part 12a of the crankshaft 12, and the engine output is output form the one end part 12a. 
Passages 14, 16 on the one and the other sides shown on the right and left in FIG. 6 are scavenging passages. These scavenging passages 14, 16 are located opposite to each other. The scavenging passage 14, 16 on the one and the other sides respectively have lower ends communicating with the crank chamber 18 and upper ends communicating with scavenging ports 20, 22 on the one and the other sides. The scavenging ports 20, 22 on the one and the other sides are opened and closed by the piston 4.
In the conventional example shown in FIG. 6, the one passage 14 and the other passage 16 are arranged opposite to each other in a directly opposite relationship. However, this is merely exemplification, and the one passage 14 and the other passage 16 are somewhat offset in a circumferential direction of a cylinder bore in some conventional examples.
The one and the other scavenging passages 14, 16 are supplied with air from upper parts thereof. In particular, the air is filled through a piston groove 24 formed in a circumferential surface of the piston 4 and the scavenging ports 20, 22 into the scavenging passages 14, 16. In the early phase of the scavenging stroke, the air stored in the scavenging passages 14, 16 is discharged through the scavenging ports 20, 22 to the combustion chamber 8. Subsequently, a fuel-air mixture pressurized in the crank chamber 18 is supplied through the scavenging passages 14, 16 and the scavenging ports 20, 22 to the combustion chamber 8.
The one scavenging passage 14 is located on the air-cooling fan side of the cylinder 6, and the other scavenging passage 16 is located on the opposite side.